Foaming methods for making cellular thermoplastic materials

ABSTRACT

Methods for reducing the density of thermoplastic materials and the articles made therefrom having similar or improved mechanical properties to the solid or noncellular material. Also disclosed are improvements to foaming methods and the cellular structures of the foams made therefrom, and methods for altering the impact strength of solid or noncellular thermoplastic materials and the shaping of the materials into useful articles.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/894,440, filed on Mar. 12, 2007, which is fully incorporated hereinexpressly by reference.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under CMS0122055 awardedby National Science Foundation. The Government has certain rights in theinvention.

BACKGROUND

Solid thermoplastic materials have many uses and applications. In almostall uses and applications, materials are chosen based on mechanicalproperties that satisfy the requirements of the use and application.Oftentimes, the constraints of the mechanical properties limit theselection of materials that are suitable. Weight is also often aconsideration that goes into the selection of the materials used in aparticular application.

Generally, a material that is lighter in weight and that possesses therequired mechanical properties will be favored over another materialwith similar properties and is higher in weight.

SUMMARY

Accordingly, in view of the above considerations, disclosed herein aremethods for reducing the density of thermoplastic materials and thearticles made therefrom having similar or improved mechanical propertiesto the solid or noncellular material. Also disclosed are improvements tofoaming methods and the cellular structure of the foams made therefrom,and methods for altering the impact strength of solid or noncellularthermoplastic materials and the shaping of the materials into usefularticles.

In one embodiment, a method for decreasing the density of athermoplastic material without substantially reducing the lighttransmissivity of the thermoplastic material is provided. The methodincludes obtaining a thermoplastic material having an initial density;and forming cells in the material that have an average cell size of 0.1nm to 100 nm to produce a thermoplastic material of lesser density thanthe initial density and having a light transmissivity of at least 20%.

In a second embodiment, a cellular thermoplastic material prepared froma solid thermoplastic material is provided. The cellular thermoplasticmaterial includes nano-sized cells having an average size of 0.1 nm to100 nm; a density lower than the density of the solid thermoplasticmaterial; and a light transmissivity of at least 20%.

In a third embodiment, a method for decreasing the density of athermoplastic material without substantially reducing the impactstrength of the thermoplastic material is provided. The method includesobtaining a thermoplastic material having an initial density and initialimpact strength; and forming cells in the material that have an averagecell size of 0.1 nm to 100 nm to produce a thermoplastic material oflesser density than the initial density and having an impact strengthsubstantially the same as or greater than the initial impact strength.

In a fourth embodiment, a cellular thermoplastic material prepared froma solid thermoplastic material is provided. The cellular thermoplasticmaterial includes nano-sized cells having an average cell size of 0.1 nmto 100 nm; a density lower than the density of the solid thermoplasticmaterial; and an impact strength substantially the same as or greaterthan the impact strength of the solid thermoplastic material.

In a fifth embodiment, a method for decreasing the density of athermoplastic material without substantially reducing the elongation ofthe thermoplastic material is provided. The method includes obtaining athermoplastic material having an initial density and an initialelongation; and forming cells in the material that have an average cellsize from 0.1 nm to 100 nm to produce a thermoplastic material of lesserdensity than the initial density and having an elongation that issubstantially the same as the initial elongation.

In a sixth embodiment, a cellular thermoplastic material prepared from asolid thermoplastic material is provided. The cellular thermoplasticmaterial includes nano-sized cells having an average cell size of 0.1 nmto 100 nm; a density lower than the density of the solid thermoplasticmaterial; and an elongation that is substantially the same as theelongation of the solid thermoplastic material.

In a seventh embodiment, a method for filtering light is provided. Themethod includes directing light at a thermoplastic material withnano-sized cells having an average cell size of 0.1 nm to 100 nm,wherein the size of the cells determines the wavelength of light that isfiltered by the thermoplastic material.

In an eighth embodiment, a light filter is provided. The light filterincludes a thermoplastic material comprising nano-sized cells having anaverage cell size of 0.1 nm to 100 nm, wherein the size of the cellsdetermines the wavelength of light that is filtered.

In a ninth embodiment, a method for making a light filter is provided.The method includes forming nano-sized cells having an average cell sizeof 0.1 nm to 100 nm in a thermoplastic material, wherein the size of thecells determines the wavelength of light that is filtered.

In a tenth embodiment, a method for providing color to a thermoplasticmaterial is provided. The method includes forming nano-sized cellshaving an average cell size of 0.1 nm to 100 nm in a thermoplasticmaterial, wherein the size of the cells determines the color of thethermoplastic material.

In an eleventh embodiment, a colored thermoplastic material is provided.The colored thermoplastic material includes nano-sized cells having anaverage size of 0.1 nm to 100 nm, wherein the size of the cellsdetermines the color of the thermoplastic material.

In the first through eleventh embodiments, the average cell size can be20 nm to 40 nm; and the thermoplastic material can be an amorphous orsemi-crystalline polymer. Representative thermoplastic materials inaccordance with the first through eleventh embodiments are disclosedherein.

In a twelfth embodiment, a method for making a cellular thermoplasticmaterial is provided. The method includes obtaining a thermoplasticmaterial impregnated with gas; placing the material on a press; applyingpressure to the material with the press; heating the press to form cellsin the material; and channeling gas away from the material through a gaschanneling means to provide a cellular material substantially free ofinternal blistering and surface deformations.

The method of the twelfth embodiment, wherein the gas channeling meansis a breather layer juxtaposed between the surface of the material andthe press.

The method of the twelfth embodiment, wherein the gas channeling meansis one or more through holes provided in the press.

The method of the twelfth embodiment, wherein the press comprises afirst platen and a second platen and the gas channeling means isprovided between a surface of the thermoplastic material and a surfaceof a platen.

The method of the twelfth embodiment, further comprising impregnatinggas into the thermoplastic material at a pressure of 1 MPa to 5 MPa.

The method of the twelfth embodiment, further comprising placing morethan one thermoplastic materials in the press and heating the more thanone thermoplastic material with the press.

The method of the twelfth embodiment, further comprising impregnatingmore than one thermoplastic material with gas.

The method of the twelfth embodiment, wherein the thermoplastic materialis a sheet having an upper surface and a lower surface and pressure isapplied on both the upper and lower surfaces of the material with thepress.

In a thirteenth embodiment, a method for making a cellular thermoplasticmaterial is provided. The method includes obtaining a thermoplasticmaterial impregnated with gas, wherein the material includes a surfacedefining a length/width plane and the material has a thickness; placingthe material on a press; applying a force to the material with the pressnormal to the surface, wherein the force produces friction between thesurface and the press to generally cause expansion in the thicknessdimension and prevent expansion in the length/width plane.

The method of the thirteenth embodiment, wherein more than onethermoplastic material is placed on the press.

The method of the thirteenth embodiment, wherein more than onethermoplastic material is saturated with gas, wherein a porous media isinterleaved between thermoplastic materials.

The method of the thirteenth embodiment, wherein the foam thermoplasticmaterial is greater than 3 mm in thickness.

The method of the thirteenth embodiment, wherein the foam thermoplasticmaterial is 6 mm in thickness or greater.

In a fourteenth embodiment, a method for making a composite structureincluding a thermoplastic foam is provided. The method includesobtaining a thermoplastic material impregnated with a gas and afacesheet; applying an adhesive on at least one surface of thethermoplastic material or the facesheet or both; placing the facesheeton the thermoplastic material; placing the gas-impregnated thermoplasticmaterial with adhered facesheet on a press; and heating the press tocure the adhesive and create a cellular structure in the thermoplasticmaterial impregnated with gas.

The method of the fourteenth embodiment, further comprising applying anadhesive to a second surface of the gas-impregnated thermoplasticmaterial or second facesheet and placing the second facesheet on thesecond surface.

The method of the fourteenth embodiment, wherein the press is heated toa temperature to cure the adhesive.

The method of the fourteenth embodiment, further comprising selectingone of saturation pressure, saturation time, or desorption time tocontrol the density of the cellular structure in the thermoplasticmaterial for the curing temperature of the adhesive.

In a fifteenth embodiment, a method for making a cellular structure isprovided. The method includes placing a first thermoplastic materialover a second thermoplastic material, wherein a surface of the firstthermoplastic material overlaps a surface of the second thermoplasticmaterial; impregnating the first and second thermoplastic materials witha gas, wherein the gas preferentially impregnates through nonoverlappingsurfaces to achieve areas of higher gas concentration closer to thenonoverlapping surfaces suitable for foaming and areas of lower gasconcentration not suitable for foaming; placing the overlapping firstand the second thermoplastic materials on a press; and heating the firstor second thermoplastic materials to cause foaming at the areas ofhigher gas concentration in the first and second thermoplastic materialsand leave the areas of lower gas concentration as solid areas in thefirst and the second thermoplastic materials.

The method of the fifteenth embodiment, further comprising bonding thefirst thermoplastic material to the second thermoplastic material sothat the foamed area of the first thermoplastic material is next to thefoamed area of the second thermoplastic material and the solid areas ofthe first and the second thermoplastic materials are the exteriorlayers.

The method of the fifteenth embodiment, further comprising trimming theedges of the first and the second thermoplastic materials.

The method of the fifteenth embodiment, further comprising placing morethan one pair of first and second overlapping thermoplastic materials ina pressure vessel and interleaving a porous material between pairs.

The method of the fifteenth embodiment, further comprising placing morethan one pair of thermoplastic materials on the press.

In a sixteenth embodiment, a cellular thermoplastic material isprovided. The cellular thermoplastic material includes micro-sized cellshaving an average cell size of greater than 1.0 μm to 100 μm, whereinthe micro-sized cells comprise cell walls; and nano-sized features inthe cell walls of the micro-sized cells, wherein the nano-sized featureshave an average size of 0.1 nm to 500 nm n.

The sixteenth embodiment, wherein the thermoplastic material is anamorphous or semi-crystalline polymer.

The sixteenth embodiment, wherein the average is 20 nm to 40 nm.

The sixteenth embodiment, wherein the nano-sized features provide openconnectivity between adjacent micro-sized cells.

The sixteenth embodiment, comprising an intrabimodal cellular structure.

In a seventeenth embodiment, a cellular thermoplastic material isprovided. The cellular thermoplastic material includes a primarystructure comprising nano-sized cells having an average size of lessthan 1 μm; and secondary micro-sized cells having an average size of 2μm to 3 μm interspersed among the primary structure.

The seventeenth embodiment, wherein the primary structure comprises themajority of the cellular thermoplastic material.

The seventeenth embodiment, comprising an interbimodal cellularstructure.

In an eighteenth embodiment, a cellular thermoplastic material isprovided. The cellular thermoplastic material includes a primarystructure comprising cells having an average size of 1 μm to 2 μm; andsecondary micro-sized cells having an average size of 10 μm to 15 μminterspersed among the primary structure.

The eighteenth embodiment, wherein the primary structure comprises themajority of the cellular thermoplastic material.

The eighteenth embodiment, comprising an interbimodal cellularstructure.

In a nineteenth embodiment, a cellular thermoplastic material isprovided. The cellular thermoplastic material includes a primarystructure comprising nano-sized cells having an average size of 0.1 nmto 100 nm; and secondary micro-sized cells having an average size ofgreater than 0.1 μm to 100 μm interspersed among the primary structure.

The nineteenth embodiment, wherein the primary structure comprises themajority of the cellular thermoplastic material.

The nineteenth embodiment, comprising an interbimodal cellularstructure.

In a twentieth embodiment, a cellular thermoplastic material isprovided. The cellular thermoplastic material includes a primarystructure comprising cells having an average size of less than 2 μm; andsecondary micro-sized cells having an average size of 2 μm to 100 μminterspersed among the primary structure.

The twentieth embodiment, wherein the primary structure comprises themajority of the cellular thermoplastic material.

The twentieth embodiment, comprising an interbimodal cellular structure.

In a twenty-first embodiment, a method for altering the impact strengthof a solid thermoplastic material is provided. The method includesobtaining a solid thermoplastic material having an initial impactstrength; treating the material under pressure to cause the material toabsorb a gas; and treating the material at a lower pressure to allowdesorption of gas from the material to produce a solid material havingan impact strength altered from the initial impact strength of the solidthermoplastic material.

In a twenty-second embodiment, a method for altering the impact strengthof a solid thermoplastic material without substantially changing thedensity of the solid thermoplastic material is provided. The methodincludes obtaining a solid thermoplastic material having an initialimpact strength and initial density; treating the material underpressure to cause the material to absorb a gas; and treating thematerial at a lower pressure to allow desorption of gas from thematerial to produce a solid material having an impact strength alteredfrom the initial impact strength and a density substantially the same asthe initial density of the solid thermoplastic material.

In the twenty-first and twenty-second embodiments, the method whereinthe thermoplastic material is an amorphous or semi-crystalline polymer.

In the twenty-first and twenty-second embodiments, the method furthercomprising treating the thermoplastic material at a pressure of 1 MPa to5 MPa.

In the twenty-first and twenty-second embodiments, the method furthercomprising treating the thermoplastic material at a lower pressure inambient atmospheric pressure.

In the twenty-first and twenty-second embodiments, the method whereinthe thermoplastic material is polyetherimide.

In the twenty-first and twenty-second embodiments, the method furthercomprising shaping the thermoplastic material during treating thematerial at a lower pressure.

In the twenty-first and twenty-second embodiments, the method whereinthe thermoplastic material comprises absorbed gas during shaping.

In the twenty-first and twenty-second embodiments, the method furthercomprising shaping the thermoplastic material after treating thematerial at a lower pressure.

In the twenty-first and twenty-second embodiments, the method whereinthe thermoplastic material is desorbed of gas during shaping.

In the twenty-first and twenty-second embodiments, the method furthercomprising placing more than one thermoplastic material in a pressurevessel and interleaving a porous material between thermoplasticmaterials.

In the twenty-first and twenty-second embodiments, the method furthercomprising assembling the solid thermoplastic material with alteredimpact strength into an article.

In the twenty-first and twenty-second embodiments, the method whereinthe thermoplastic material is a thermoplastic urethane, thermoplasticelastomer, polyethylene naphthalate, polyetherimide,polyetheretherketone, polyphenylene, sulfone, polyamide-imide,polysulfone, polyphenylsulfone, polyethersulfone, polyphthalamide,polyarylamide, polyphenylene sulfide, cyclic olefin copolymer,polyphthalate carbonate, polycarbonate, polyvinylidene chloride,polyurethane, polyphenylene oxide, poly(acrylonitrile-butadiene-styrene), polymethylmethacrylate, crosslinkedpolyethylene, polystyrene, styrene acrylonitrile, polyvinyl chloride,polybutylene tercphthalatc, polyethylene terephthalate,polyoxymethylene, polyacetal, polyamide, polyolefin, polyethylene,polypropylene.

In the twenty-first and twenty-second embodiments, the method whereinthe impact strength is greater than the initial impact strength.

In the twenty-first and twenty-second embodiments, the method whereinthe impact strength is less than the initial impact strength.

In all embodiments above, the thermoplastic material can be athermoplastic urethane, thermoplastic elastomer, polyethylenenaphthalate, polyetherimide, polyetheretherketone, polyphenylene,sulfone, polyamide-imide, polysulfone, polyphenylsulfone,polyethersulfone, polyphthalamide, polyarylamide, polyphenylene sulfide,cyclic olefin copolymer, polyphthalate carbonate, polycarbonate,polyvinylidene chloride, polyurethane, polyphenylene oxide, poly(acrylonitrile-butadiene-styrene), polymethylmethacrylate, crosslinkedpolyethylene, polystyrene, styrene acrylonitrile, polyvinyl chloride,polybutylene terephthalate, polyethylene terephthalate,polyoxymethylene, polyacetal, polyamide, polyolefin, polyethylene,polypropylene.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a flow diagram of one embodiment of a method in accordancewith the present invention;

FIG. 2 is a flow diagram of a method in accordance with one embodimentof the present invention;

FIG. 3 is a flow diagram of a method in accordance with one embodimentof the present invention;

FIG. 4 is a diagrammatical illustration of a heated press shown beingused in a prior art foaming method;

FIG. 5 is a diagrammatical illustration of a heated press shown beingused in a foaming method in accordance with one embodiment of thepresent invention;

FIG. 6 is a diagrammatical illustration of a heated press shown beingused in a foaming method in accordance with one embodiment of thepresent invention;

FIG. 7 is a diagrammatical illustration of a heated press shown beingused in a foaming method in accordance with one embodiment of thepresent invention;

FIG. 8 is a diagrammatical illustration of a pressure vessel being usedto gas saturate thermoplastic materials interleaved with porousmaterials to be used in a foaming method in accordance with oneembodiment of the present invention;

FIG. 9 is a diagrammatical illustration of a heated press shown beingused in a foaming method in accordance with one embodiment of thepresent invention;

FIG. 10 is a diagrammatical illustration of a heated press shown beingused in a foaming method in accordance with one embodiment of thepresent invention;

FIG. 11 is a diagrammatical illustration of thermoplastic materials tobe used in a foaming method in accordance with one embodiment of thepresent invention;

FIG. 12 is diagrammatical illustration of a pressure vessel being usedto partially gas saturate pairs of thermoplastic materials to be used ina foaming method in accordance with one embodiment of the presentinvention;

FIG. 13 is diagrammatical illustration of a heated press being used in afoaming method in accordance with one embodiment of the presentinvention;

FIG. 14 is diagrammatical illustration of partially foamed thermoplasticmaterials to be used in a method in accordance with one embodiment ofthe present invention;

FIG. 15 is diagrammatical illustration of partially foamed thermoplasticmaterials adhered to each other to produce a composite material inaccordance with one embodiment of the present invention;

FIG. 16 is a graph plotting the relative density of microcellular andnanocellular foams as a function of foaming temperature;

FIG. 17 is a scanning electron micrograph of a foam containingnano-sized cells made in accordance with one embodiment of the presentinvention;

FIG. 18 is a scanning electron micrograph of a foam containingmicro-sized cells made in accordance with one embodiment of the presentinvention;

FIG. 19 is a scanning electron micrograph of a foam containingmicro-sized cells with nano-sized features on the cell walls in anintrabimodal cellular structure in accordance with one embodiment of thepresent invention;

FIG. 20 is a scanning electron micrograph of a foam containingmicro-sized cells with nano-sized features on the cell walls in anintrabimodal cellular structure in accordance with the presentinvention;

FIG. 21 is a scanning electron micrograph of a foam containing amajority of smaller cells with interspersed larger cells in aninterbimodal cellular structure in accordance with one embodiment of thepresent invention;

FIG. 22 is a scanning electron micrograph of a foam containing amajority of smaller cells with interspersed larger cells in aninterbimodal cellular structure in accordance with one embodiment of thepresent invention;

FIG. 23 is a schematic diagram of a device for testing lighttransmissivity in accordance with one embodiment of the presentinvention;

FIG. 24 is a graph of light transmissivity of microcellular andnanocellular foams as a function of void fraction;

FIG. 25 is a graph of the impact strength of virgin polyetherimide;microcellular foam polyetherimide, nanocellular foam polyetherimide, andsaturated and desorbed polyetherimide;

FIG. 26 is a graph of the stress/strain curve of virgin polyetherimideand saturated and desorbed polyetherimide;

FIG. 27 is a graph of the stress/strain curve of microcellular andnanocellular polyetherimide foam at a relative density of 75%;

FIG. 28 is a graph of the stress/strain curve of microcellular andnanocellular polyetherimide foam at a relative density of 90%;

FIG. 29 is a graph of the stress/strain curve of microcellular andnanocellular polyetherimide foam at a relative density of 82.5%;

FIG. 30 is a graph of the tensile strain at break of virginpolyetherimide, saturated and desorbed polyetherimide, microcellularpolyetherimide foam, and nanocellular polyetherimide foam as a functionof void fraction;

FIG. 31 is a bar graph of the impact energy of solid polyetherimide thathas been saturated and desorbed of gas at a high and low pressure;

FIG. 32 is a bar graph of the impact energy of solid polycarbonate thathas been saturated and desorbed of gas at a high and low pressure; and

FIG. 33 is a bar graph of the impact energy of solid high impactpolystyrene that has been saturated and desorbed of gas at variouspressures.

DETAILED DESCRIPTION

Microcellular and nanocellular thermoplastic materials are a group offoams that have a high concentration of small diameter cells. Themicrocellular range typically refers to cell diameters between greaterthan 1 μm to 100 μm. Nanocellular polymers are characterized by celldiameters in the sub-micrometer range, typically 0.1 nm to 100 nm.Foams, in general, offer density reductions over unfoamed solidmaterial, thus resulting in a reduction of raw material for the samepart or component when a foam material is used in place of the solidmaterial.

In one aspect, embodiments of the present invention relate to methodsfor reducing the density, i.e., the weight of a thermoplastic material,yet the methods substantially maintain or even improve the desirablemechanical properties of the thermoplastic material in the solid form.

Referring to FIG. 1, a representative flow diagram of a method forreducing the density of solid thermoplastic materials withoutsubstantially lowering selected desirable mechanical properties of thesolid thermoplastic materials is illustrated. The method provides thelower density and substantially the same mechanical properties byforming nano-sized cells in the solid thermoplastic material. Theaverage size of the nano-sized cells is 0.1 nm to 100 nm. Anothersuitable range is 20 nm to 40 nm. The method includes block 100. Inblock 100, a solid thermoplastic material with a desirable property isobtained. The solid thermoplastic material has an initial density and aninitial value of a desirable property. A desirable property can be, butis not limited to, light transmissivity, impact strength, and tensileelongation (strain). A thermoplastic material of lower density is usefulin generally all applications where the solid thermoplastic material isuseful, but because of the lower density, the weight and the amount ofmaterial used is correspondingly reduced without reduction of thedesirable property of the solid thermoplastic material.

From block 100, the method enters block 102. In block 102, the solidthermoplastic material is subjected to a foaming process. Varioussuitable embodiments of a foaming process will be described below inconnection with FIG. 3. Foaming processes can be solid-state foamingprocesses or liquid-state processes.

From block 102, the method enters block 104. In block 104, the methodproduces a nanocellular thermoplastic material that is lower in densityto the initial density of the solid thermoplastic material and has asubstantially similar or improved property as the solid thermoplasticmaterial. The foamed thermoplastic material that is produced in block106 can be used as a substitute for the solid thermoplastic material tomake a part or a component that would conventionally be made from thesolid thermoplastic material of block 102.

Referring to FIG. 2, a representative flow diagram of a method suitableto be used as the foaming method in block 104 of FIG. 1 is illustrated.The method of FIG. 2 is a solid-state foaming method. However,liquid-state methods may also be used in place of the solid-statefoaming method. Solid-state foaming is a process by which foaming occurswhile the polymer remains in the solid state throughout the process.This process differs from other standard polymer foaming processesbecause the polymer is not required to be in a molten state.

The method includes block 202. In block 202, a thermoplastic material isobtained. The thermoplastic material can be a solid material. Generally,at the beginning of the method, the thermoplastic material is inequilibrium with the surrounding room temperature and atmosphericpressure, and the material is referred to as “unsaturated.” Anythermoplastic material that can absorb a gas is suitable to be used inembodiments disclosed herein. From block 202, the method enters block204. In block 204, the thermoplastic material is treated at an elevatedpressure to cause the thermoplastic material to absorb gas. A suitablegas for use in the method is carbon dioxide at pressures in the range ofabout 1 MPa to about 5 MPa. However, the pressure may vary depending onthe solid thermoplastic material and the gas used. The treatment ofthermoplastic material in block 204 may be carried out in a pressurevessel, which is sealed, and then the thermoplastic material is exposedto a high pressure inert gas such as, but not limited to, carbon dioxide(CO₂) at room temperature within the pressure vessel. The high pressuregas will then start to diffuse into the thermoplastic material overtime, filling the material's free intermolecular volume. The gas willcontinue to saturate the material until equilibrium is reached. As usedherein, “saturate” or any derivation thereof means “fully saturated”unless indicated otherwise. Partially saturated means that certainsections have sufficient gas absorbed for nucleation and bubble growthfor the foaming temperature. At equilibrium, the sample is said to be“fully saturated.” The thermoplastic material can be any shapedesirable. However, a sheet is most often used because the time requiredto provide a constant gas concentration throughout the thickness of thematerial can be lengthy. More than one sheet can be placed in thepressure vessel to saturate more than one sheet at a time. If the sheetsare stacked on top of one another, a porous material is interleavedbetween sheets to allow the gas to saturate from all sides of the sheetincluding the side that is next to an adjacent and lower sheet. See, forexample U.S. Pat. No. 5,684,055, to Kumar et al., incorporated hereinexpressly by reference.

Saturation pressures, saturation times, desorption times, and foamingtemperature can be varied to effect the type of foam that is producedfrom the gas-saturated thermoplastic material. At a given foamingtemperature, a lower saturation pressure produces a foam of higherdensity and a higher saturation pressure produces a foam of lowerdensity. Higher foaming temperatures produce foams of lower density. Ata given density, a higher saturation pressure produces nano-sized cellsand a lower saturation pressure produces micro-sized cells. Arepresentative thermoplastic material discussed throughout thisapplication is polyetherimide. Saturation times to reach equilibrium mayvary with the pressure. At a saturation pressure of 1 MPa,polyetherimide produces foams having micro-sized cells, and at asaturation pressure of 5 MPa, polyetherimide produces foams havingnano-sized cells. At a saturation pressure of 4 MPa, polyetherimideproduces foams having both nano-sized cells and micro-sized cells. Fordifferent polymers, the absorption times may vary. The requiredabsorption times to reach the fully saturated condition forthermoplastic materials other than polyetherimide can readily bedetermined by experimentation. Furthermore, the equilibrium gasconcentration in milligrams of carbon dioxide per grams ofpolyetherimide also varies with saturation pressure. Generally, thethermoplastic material can be determined to be fully saturated when theconcentration of gas over time is essentially constant and does notvary. In other words, the slope of a line of a plot of gas concentrationon the ordinate and time on the abscissa is essentially zero.

Referring to FIG. 2, from block 204, the method enters block 206. Inblock 206, the thermoplastic material that is fully saturated with gasis treated at a lower pressure than the saturation pressure to allow thethermoplastic material to undergo desorption of gas. Desorption of somegas is desirable in some circumstances, for example, to avoid thecreation of cellular structure in some areas of the thermoplasticmaterial, such as at the surfaces. Desorption of the thermoplasticmaterial can occur when the high pressure gas is vented from thepressure vessel or the saturated material is removed into ambientatmospheric pressure. During block 206, the fully saturated material isremoved from the saturation pressure to an environment of lowerpressure, so that the material is thermodynamically unstable, meaningthat the material is supersaturated with gas and is no longer atequilibrium with the surrounding environment. The material will start todesorb gas from its surface into the surrounding environment. Desorptionof gas from the surface will give the foam an integral solid skinbecause the desorbed skin will not have sufficient gas to foam duringthe heating step. The thickness of the solid skin at the surface can beincreased by allowing for greater desorption times.

Referring to FIG. 2 again, from block 206, the method enters a heatingstep to create foam. In the illustrated embodiment, the method may enterone of two solid-state foaming processes. However, any other suitableheating process may be used. A first solid-state foaming process isfoaming by heating the thermoplastic material in a heated liquid bath,block 208. In a second foaming process, the thermoplastic material canbe heated in a press to foam the thermoplastic material, block 210. Thelatter process has the advantage of creating flat foams. There are aplurality of variations of the heated press foaming method that will bedescribed below in more detail. While the heated liquid bath and heatedpress method are illustrated, it should be readily appreciated thatother methods for heating a solid may be used, such as, but not limitedto, a flotation/impingement air oven, or an infrared oven.

In either block 208 or block 210, heating transforms sections of thethermoplastic material from a solid to a cellular structure. Foamingoccurs where conditions of gas concentration and temperature aresufficient. When either the gas concentration or temperature areinsufficient, foaming does not occur and the thermoplastic materialremains a solid, such as at the surfaces from which some amount of gaswas allowed to desorb. In block 208, the saturated material is placedinto a heating environment, such as a hot liquid bath. The heated liquidbath, block 208, uses a reservoir containing a hot medium, such as oil,heated to a particular temperature. The bath raises the temperature ofthe material above the glass transition temperature of the polymer-gassystem. Above the glass transition temperature, the material will softenand the polymer matrix will begin to crack, providing areas where thesaturated gas will begin to fill in; and as heating occurs, thesolubility of the gas decreases, and in the areas where the polymermatrix has cracked, nucleation will occur and bubble growth will begin.The main variable that is controlled is the foaming temperature. Thefoaming temperature will accurately control the final density andcellular structure of the foamed material. After the sample has beenallowed to foam for a controlled time, the sample is removed from theheating bath and allowed to cool to room temperature. An alternative tothe hot liquid bath is to heat the material in a press.

Block 210 of FIG. 2 may use a press having a first and a second platen.In one embodiment, the first and second platens can be an upper and alower platen. The surface of the upper and lower platens are flat. Thisallows for the creation of foams with flat surfaces. However, shapedplatens can be used to mold the thermoplastic material into anydesirable shape. The platens are preferably heated to the desiredtemperature with the platens in the closed position. Once the platensare at temperature, the saturated thermoplastic material is placed inthe press between the upper and the lower platens. The platens are thenforced to close against the upper and lower surfaces of thethermoplastic material so that the lower surface of the upper platentouches the upper surface of the thermoplastic material and the uppersurface of the lower platen touches the lower surface of thethermoplastic material. The thermoplastic material can be foamedaccording to various embodiments of the press foaming technique asillustrated in FIGS. 4 through 6.

After creating a cellular thermoplastic material, it may be desirable totest the mechanical properties to determine whether the cellularthermoplastic material has substantially the same or improved propertiesas compared to the solid (or unfoamed) version of the thermoplasticmaterial. Generally, for mechanical testing, most if not all theresidual gas is allowed to desorb from the cellular thermoplasticmaterial. A suitable method for determining the minimum length of timerequired for desorption for mechanical testing may be, for example, byplotting gas concentration on the ordinate versus time on the abscissato determine when the concentration does not substantially lessen overtime. Desorption times for other polymers may vary to that ofpolyetherimide. Furthermore, during the foaming process, gas is releasedfrom the thermoplastic material to create the cellular structure. Inaddition, desorption of gas through a foam is faster than through asolid material. Accordingly, cellular thermoplastic materials will haveless residual gas than the solid thermoplastic material if both areallowed to desorb gas for the same period of time. Alternatively, thecellular thermoplastic materials can be desorbed in a vacuum chamber tospeed up the desorption process. Depending on size, such as thickness,the desorption times may vary.

Referring to FIG. 4, a conventional heated press foaming technique isillustrated. A first platen 402 and a second platen 404 face therespective upper and lower surface of a thermoplastic material 410. Afirst metal shim 406 is placed between the lower surface of the upperplaten 402 and the upper surface of the lower platen 404. A second metalshim 408 is placed between the lower surface of the upper platen 402 andthe upper surface of the lower platen 404. In this way, the upper andlower platens 402 and 404 are prevented from closing beyond thepredetermined height or thickness dictated by the dimensions of shims406 and 408. One or both platens 402 and 404 can be forced in thedirection of the arrows designated “F” to press against shims 406 and408. The force “F” is sufficient to prevent the foam 410 from exceedingthe thickness of the shims 406 and 408. One or both platens 402 and 404can also be heated as indicated by the symbol “h” designating theapplication of heat to the platens 402 and 404. In this technique, priorto the material 410 foaming, the material begins by only touching oneplaten, generally the lower platen 404, so that heat to the lowersurface of the material 410 is transferred by conduction. Beforefoaming, the upper surface of the material 410 is not touching the lowersurface of the upper platen 402 so that initially heat to the uppersurface of the material 410 is transferred mainly by radiation and/orconvection from the upper platen 402. As the material 410 foams, thethickness of material 410 increases to the dimensions as determined bythe shims 406 and 408, but the material 410 does not force the platens402, 404 apart. Therefore, the thickness of the foam material 410 can bedetermined by selecting the appropriate shim dimension. The foamingprocess is thus constrained to conform to the flat geometry with apreset thickness, hence the term “constrained” foaming. In a variationof this process, the platens can be replaced by surfaces of a mold tocreate desired shapes other than a flat geometry. An advantage of usinga constrained foaming process is that it is easy to create an integralskin of desired thickness by using desorption time as a processvariable. As desorption time increases, the thickness of the integralskin without a cellular structure increases. The time required for gasto desorb out of the fully saturated thermoplastic material sheet canrange from a few hours to days depending on the desired skin thickness.The method of FIG. 4 achieves flat surfaces by constraining the foam inthe thickness direction, which then forces the cellular thermoplasticfoam to grow in the length/width plane.

In an alternate embodiment illustrated in FIG. 5, the shims 406 and 408can be omitted. In this alternative technique, the upper platen 502 andthe lower platen 504 are allowed to touch the respective upper and lowersurfaces of the material 510 before foaming so that heat transferredfrom the platens 502, 504 to the material 510 is by conduction on boththe upper and the lower surfaces. A force is maintained by the platens502, 504 against the material 510, and as the material 510 foams, thematerial 510 expands and pushes against the upper platen 502 and thelower platen 504 and forces the upper platen 502 and the lower platen504 apart.

While the constrained foaming method as illustrated in FIG. 4 issuitable for the creation of foams, the use of the shims 406 and 408 canlead to internal blistering. A method using the heated press asillustrated in FIG. 5 may be used to eliminate some of the internalblistering caused by the use of shims, but the method using the heatedpress of FIG. 5 may result in large surface deformations.

The internal blistering and the surface deformations may be reduced bythe use of a method using the apparatus illustrated in FIG. 6. In FIG.6, a press is illustrated having an upper platen 602 and a lower platen604. The upper platen 602 and the lower platen 604 are configured suchthat a force can be applied in the direction indicated by the arrowshaving the designation “F” to one or both platens 602 and 604.Similarly, heat designated by the letter “h” may be applied to one orboth platens 602 and 604. In the embodiment illustrated in FIG. 6, theshims 406 and 408 of FIG. 4 are not used. A breather layer 612 isinterposed between the upper surface of the thermoplastic material 610and the lower surface of the upper platen 602. A second breather layer614 is interposed between the lower surface of the material 610 and theupper surface of the lower platen 604. The purpose of breather layers612 and 614 is to allow desorbed gas to escape from the thermoplasticmaterial 610 as the material is foaming. In particular, as thethermoplastic material begins to off gas, the gas is allowed to escapethrough the breather layers 612 and 614.

An alternative of the embodiment illustrated in FIG. 6 is the embodimentillustrated in FIG. 7.

FIG. 7 illustrates an upper platen 702, a lower platen 704, wherein bothupper and lower platens 702 and 704 are configured such that a force “F”can be applied to close one or both of the platens against athermoplastic material 710 as it is being foamed. However, unlike theembodiment of FIG. 6 that uses a first and second breather layer 712 and714, the embodiment of FIG. 7 eliminates the need for breather layers,and instead includes through bores 716 in the upper platen 702 andthrough bores 718 in the lower platen 704. The through bores 716 and 718are functionally equivalent to the breather layers 612 and 614 of FIG. 6in that the through bores 716 and 718 allow for the escape of gas fromthe thermoplastic material 710 such that internal blistering and surfaceindentations are reduced.

In one embodiment of a method, the device illustrated in FIG. 5, can beused to control the thickness of the cellular thermoplastic material510.

The force “F” applied by one or both platens 502 and 504 is normal tothe thermoplastic material's surfaces and, thus, creates frictionalforces “f” at the interface between the heated platens 502, 504 and thethermoplastic material 510 that resists the expansion of thethermoplastic material 510 against the frictional forces “f.” In theillustration, the thermoplastic material 510 has a thickness dimensionand a length/width, or the “in-plane” dimension. The frictional force atthis interface can be defined asf=μF  (1)

-   -   where, f is the frictional force        -   μ is the co-efficient of friction between heated platens and            foaming thermoplastic sheet        -   F is the normal force applied on the foaming plastic sheet            by the heated platen

For a given μ, as the normal force “F” increases, the frictional forcealso increases. This increasing frictional force restricts the in-planeexpansion of the foaming thermoplastic sheet 510 in the length/widthdimension. Due to this restriction on the in-plane growth, most of thefoam growth then occurs in the thickness dimension. The foaming sheet510 pushes against the platens 502, 504 overcoming the normal force andcontinues to grow in thickness. For a given set of processing conditionsincluding saturation pressure, desorption time, and foaming temperature,the final thickness of the cellular thermoplastic sheet 510 can bevaried by varying the normal force applied during foaming. Therefore,one method includes the step of controlling the normal force sufficientto increase the frictional forces to prevent growth in the length/widthdimension, but the normal force is insufficient to prevent increases inthe thickness of the microcellular plastic sheet 510. There is an upperlimit of the normal force beyond which the foaming cellularthermoplastic sheet can no longer support the compressive normal force.This may lead to either of the two conditions: (1) the driving forcebehind foaming will overcome the frictional force causing in-planeexpansion of the thermoplastic sheet, or (2) the foam structurecollapses due to cell wall failure.

FIGS. 8 and 9 show another embodiment illustrating a method in which astack of gas-saturated thermoplastic sheets 810 a, b, c, and d can allbe foamed at once between the heated platens 902 and 904. Foaming inthis stacked manner can create thick composite cellular sheets withdesirable density and thickness. It is possible to create multi-layeredcomposite cellular structures by foaming and bonding more than onegas-saturated thermoplastic sheet in one step according to the followingmethod.

The method includes saturating more than one thermoplastic sheet at highpressure with gas in a pressure vessel 802 as illustrated in FIG. 8. InFIG. 8, thermoplastic sheets 810 a, b, c, and d are interleaved with aporous media 811 a, b, c, and d, such as paper towels, to provide ameans for allowing the gas to saturate the surfaces of sheets adjacentto one another between the thermoplastic sheets. The one or more of thethermoplastic sheets 810 a, b, c, and d are placed in the pressurevessel 802 and a high pressure gas, such as carbon dioxide, isintroduced to the vessel 802. After achieving full saturation, the oneor more sheets are removed from the pressure vessel 802, and are allowedto desorb for the desired desorption time. After the predetermineddesorption time has elapsed, the gas-saturated thermoplastic sheets 810a, b, c, and d are transferred to the press between the heated platens902 and 904 as illustrated in FIG. 9. This can be done either onethermoplastic sheet at a time or more than one thermoplastic sheetswithout the porous interleaved media. An adhesive may be used betweenthe sheets to bond the sheets into a composite cellular structure. Theheated platens 902 and 904 may be at the desired foaming temperature.The hydraulic press is operated to apply a normal force to the platens.The normal force is chosen depending on the desired final foamthickness.

Another embodiment for the use of the device of FIG. 5 is a method forcreating a multi-layered cellular structure that has one or morefacesheets made from the same or different material and a core foammaterial. Currently, composite sandwich panels are constructed in two ormore steps. The composite facesheets and foam (or honeycomb) core aremanufactured separately. Then, the composite facesheets are bonded tothe two surfaces of the foam (or honeycomb) core with adhesive resins.This assembly is then cured at elevated temperatures to achieve alightweight composite sandwich structure with excellent stiffness andstrength characteristics. In addition to the multiple manufacturingsteps, and due to the porous surface of the foam (or honeycomb) coreused currently, the amount of adhesive resin used to bond the facesheetsuniformly to the surface of the foam core is wasteful. Adhesive resin isused to fill up the pores on the foam (or honeycomb) core surfaces so asto create a flat continuous layer of adhesive resin to which thefacesheets are attached. This extra adhesive resin also increases theoverall weight of the sandwich panel.

One embodiment is a method for making a composite structure with a foamcore and at least one facesheet as illustrated in FIG. 10.Conventionally, structures having a foam core and facesheets wereassembled after independently forming the foam core and the facesheetseparately. The facesheet would then be normally adhered with anadhesive to the foam core. However, because of the foam or honeycombcore that is used, adhesive is wasted on covering the entire cellularsurface of a foam or honeycomb structure. The disclosed method can beused to provide lighter composite sandwich panels due to a reduction inthe amount of adhesive resin. The adhesive used is less because theadhesive is added to the core and/or facesheet before the core isconverted into a foam, not after, as in the conventional method. Themethod involves using a cellular thermoplastic material 1010 as a corein place of the conventional foam or honeycomb core used currently. Thedensity of the cellular thermoplastic material 1010 can be created witha density that is substantially the same as that of the foam cores usedcurrently. As previously described, a stack of thermoplastic sheets canbe saturated with carbon dioxide gas in a high-pressure environment byinterleaving a porous material between sheets to allow gas to penetratefrom all surfaces of the sheets. After saturation of the thermoplasticsheets, they are removed from the pressure vessel and allowed to desorbgas for a few minutes so as to enable forming a thin integral solid skinwhen the sheet is foamed in the heating step. After the predetermineddesorption time has elapsed, one or more facesheets 1012 and 1014 arebonded to the upper and lower surfaces of each gas-saturatedthermoplastic sheet 1010 using a thin layer of adhesive resin 1016 and1018. The assembly is then placed between the heated platens 1002 and1004 of a hydraulic press and the press is controlled sufficient to keepthe composite substantially flat. The platens 1002 and 1004 are heatedto the curing temperature of the adhesive resin. The curing temperatureis therefore the foaming temperature of the gas-saturated thermoplasticsheets. The assembly is held at this temperature to allow the adhesiveto cure and the gas-saturated sheet to foam in one step. The smoothintegral skin that is formed because of desorption allows using reducedamounts of adhesive resin between the facesheets 1012 and 1014 and thecellular foam core 1010. Since the foaming temperature is also thecuring temperature of the adhesive resin and cannot be varied, thedensity of the cellular foam core 1010 can be controlled by varying theother processing variables, such as saturation pressure, saturationtime, desorption time, etc.

Another embodiment is a method of forming multi-layered panels. Thepanels have a foam core at the center and a solid thick skin surface.Conventionally, to make such a panel, a single, monolithic thermoplasticmaterial was used. Because a foam core was desired, the thermoplasticmaterial was fully saturated to achieve a uniform gas concentrationthroughout the material, including the center. The thermoplasticmaterial was then allowed to desorb gas from the upper and lowersurfaces for a predetermined length of time that would result in thesolid skin surface of desired thickness. However, the time periods forfull saturation to reach equilibrium and desorption required longperiods of time. The disclosed method uses a first and second thinnersheet of thermoplastic material to lessen the amount of time requiredfor absorption.

Referring to FIG. 11, in one embodiment of making a multi-layered panel,first 1102 and second 1104 thermoplastic sheets are stacked togetherwith one of their major surfaces overlapping and in contact with oneanother. During saturation, only one of the two major surfaces of eachof the thermoplastic sheets 1102 and 1104 is exposed to a high-pressurecarbon dioxide gas environment as shown in FIG. 12. The porousinterleaved materials described earlier are not used between pairs ofsheets, thus, preventing absorption of gas via the overlapping surfacesbecause the overlapping surfaces are not fully exposed to the gas. Sincethe overlapping surfaces are kept in contact, the gas diffuses into thethermoplastic sheets 1102 and 1104 mainly through the nonoverlappingsurfaces exposed to the high-pressure environment. In the case ofmultiple pairs as illustrated in FIG. 12, porous interleaved materials1108 a, 1108 b, and 1108 c are used and placed between the pairs ofsheets to allow gas to absorb on the surfaces between pairs. The time inthe pressure vessel 1201 is chosen as needed to attain minimum gasconcentration required for foaming in approximately half of the startingsheet thickness (denoted by lattice in the illustration). It isnoteworthy that in FIG. 12, the overlapping surfaces in each pair ofsheets has not reached the minimum gas concentration necessary forfoaming. In FIG. 12 all around the border areas, with the exception ofoverlapping surfaces, a lattice representing the regions of the solidthermoplastic sheets having the minimum gas concentration available forfoaming at the predetermined foaming temperature is shown. The centerareas are regions which do not have the minimum gas concentrationrequired for foaming. Hence, during the foaming step, the border areasof the thermoplastic sheets will have bubble nucleation and growth whilethe center areas of the sheets will remain solid. Once the gasconcentration profile is achieved, the pairs of stacked sheets areremoved from the pressure vessel 1201 and are transferred (optionally instacked pairs) to the hydraulic press as shown in FIG. 13. The platens1302 and 1304 of the press are set at a temperature that causes foamingin the border areas with minimum gas concentration to cause bubblenucleation and growth in those areas, while the center areas with lowerthan the minimum gas concentration remain solid. The stacked pair ofsheets 1102 and 1104 is put in the heated press such that the surfaceswith the minimum gas concentration required for foaming are in contactwith the platens 1302 and 1304. After the foam growth is completed inboth the sheets 1102 and 1104, the stacked pair of partially foamedsheets 1102 and 1104 is removed from the press. The edges at the ends ofboth of the sheets 1102 and 1104 may be trimmed as shown in FIG. 14leaving a foamed material having one side with cellular structure andone side of solid material. The trimmed sheets 1102 and 1104 are thenbonded via an adhesive or mechanical fastener to each other in a mannersuch that the layer of foam of each sheet 1102 and 1104 faces each otherand are at the center of the multi-layered panel, and the solid layersare the exterior surfaces of the multi-layered panel as shown in FIG.15. In the disclosed method, during the saturation step, gas is allowedto partially saturate the thermoplastic sheets to create the desiredskin thickness instead of allowing for full saturation, followed by longdesorption time. Thus, this method decreases the time required forproducing a multi-layered panel having a foam core with solid thickskin.

The methods disclosed in association with FIGS. 5-15 can be used tocreate thick cellular sheets. In some embodiments, the sheets can begreater than 3 mm and in other embodiments, the sheets can be at least 6mm thick. A heated press provides easier control and faster productionrates in the manufacturing of cellular thermoplastic sheets that havepotential applications in the construction industry, automobile and boatmanufacturing, and other load-bearing applications.

As disclosed herein, controlling the temperature during the foamingprocess allows for controlling the density of the foamed thermoplasticmaterial. An advantage of reducing the density is the reduction inweight and material. Thus, parts and components can be built with lessmaterial and weigh less than their solid counterparts. Referring to FIG.16, a graph illustrating the relative density of a polyetherimide foamas a function of temperature for three saturation pressures isillustrated. FIG. 16 illustrates that above a certain temperature, therelative density becomes unpredictable. For a saturation pressure of 1MPa, the maximum temperature appears to be approximately 210° C. Thelowest relative density at a saturation pressure of 1 MPa is slightlybelow 0.3. For a saturation pressure of 5 MPa, the highest temperatureappears to be approximately 180° C. The lowest relative density for asaturation pressure of 5 MPa is approximately 0.55. Relative densityvaries from about 0.3 to less than 1.0. While FIG. 16 is a graph forpolyetherimide, it is generally believed that other materials willfollow similar trends.

Further, as disclosed above in connection with FIG. 1, creatingnano-sized cells in a thermoplastic material that have an average sizeof from 0.1 nm to 100 nm, or in the range of 20 nm to 40 nm producesfoams that are lower in density to the initial density of the material,but retain some of the desirable properties of the solid thermoplasticmaterial. Among the desirable properties are light transmissivity,impact strength, and tensile elongation. The procedures for determiningdensity, light transmissivity, impact strength, and tensile elongation(strain) are described in the EXAMPLES section below.

Accordingly, respective methods for decreasing the density of athermoplastic material without substantially reducing the lighttransmissivity, impact strength, and tensile elongation of the materialare disclosed that introduce nano-sized cells having an average size of0.1 nm to 100 nm into the thermoplastic material. While tensileelongation is the property that is tested in the EXAMPLES section below,it is to be appreciated that tensile elongation is representative of amaterial's ability to bend without breaking. Therefore, the methodsdisclosed herein not only result in sustained tensile elongation ofmaterials, but also when the material is both under compression andtension, such as when a material is being bent. In this case, thematerial undergoes a compressive force on one side and tension on theopposite side. The disclosed method, therefore, produces a material thatalso has a higher bending ability without break than the solid material.Therefore, as used herein, “elongation” is not limited to elongationoccurring solely under tensile strain, and may include elongation whenunder tension and compression, such as when bending.

Furthermore, in addition to providing significant light transmissivity,nano-sized cells also have the ability to transmit or deflect differentwavelengths of light. For example, in the size of 20 nm to 40 nm, thenano-sized cells can scatter blue light and allow red light to passthrough, which has the effect of coloring the material a blue color.Accordingly, by adjusting the cell size in the nano-size range of 0.1 nmto 100 nm, color can be added to the material without introducingpigments in the material.

Because of the ability of nano-sized cells to block or deflect certainwavelengths of light, a light filter can be constructed from athermoplastic material by forming nano-sized cells of an average cellsize of 0.1 nm to 100 nm, wherein the size of the cells determines thewavelength of light that is filtered by the thermoplastic material. Thethermoplastic light filter with nano-sized cells can be used in a methodto block light of a certain wavelength.

Referring to FIG. 3, another embodiment of a method for altering theimpact strength of a solid thermoplastic material is illustrated. Themethod includes block 302. In block 302, a solid thermoplastic materialis obtained. The solid thermoplastic material has an initial impactstrength. From block 302, the method enters block 304. In block 304, thethermoplastic material is treated at an elevated pressure to cause gasto be absorbed in a thermoplastic material. From block 304, the methodenters block 306. In block 306, the thermoplastic material is treated ata lower pressure to allow gas to desorb from the thermoplastic material.From block 306, the method enters block 308. In block 308, a solidthermoplastic material having an altered impact strength different thanthe initial impact strength is produced by the treatment in blocks 304and 306. When the thermoplastic material is treated at a low pressure onthe order of 1 MPa, the impact strength is decreased compared to theinitial impact strength, and when the thermoplastic material is treatedat a high pressure on the order of 5 MPa, the impact strength isincreased compared to the initial impact strength. Whether the impactstrength is increased or decreased may vary with the saturationpressure. Optionally, the method further includes block 310. In block310, the solid thermoplastic material having an altered impact strengthdifferent than the initial impact strength can be used to form acomponent or a part that can replace the normal thermoplastic materialwith initial impact strength. In one embodiment, block 310 is performedon the thermoplastic material that is desorbed according to block 306.For example, forming a component may involve shaping the desorbedthermoplastic material in a cold pressing technique, or by applying amild heat treatment. In another embodiment, forming a component or partmay be performed simultaneously with block 306, and while thethermoplastic material is undergoing desorption of gas. For example, thethermoplastic material may be shaped by a cold pressing technique duringblock 306, or by applying a mild heat treatment. The material that istreated by the methods disclosed herein in connection with FIG. 3 may beany shape and/or size. For example, a component of an aircraft or anyland vehicle may be treated after being formed. As disclosed above, thesaturation pressure may determine whether the treatment results inincreased or decreased impact strength. Referring to FIG. 31, the impactstrength of solid polyetherimide saturated at 1 MPa and desorbed has animpact strength lower than the impact strength of the standard material.The impact strength of solid polyetherimide saturated at 5 MPa anddesorbed has an impact strength greater than the impact strength of thestandard material. Referring to FIG. 32, the impact strength of solidpolycarbonate saturated at 1 MPa and desorbed has an impact strengthgreater than the impact strength of the standard material. The impactstrength of solid polycarbonate saturated at 4 MPa and desorbed has animpact strength greater than the impact strength of the standardmaterial, but less than the solid polycarbonate saturated at 1 MPa. Theimpact strength of solid polycarbonate saturated at 7 MPa and desorbedhas an impact strength greater than the impact strength of the standardmaterial and the solid polycarbonate saturated at 4 MPa, but less thanthe solid polycarbonate saturated at 1 MPa. Polycarbonate (PC) is awidely used plastic especially for impact applications, such as sunglasslenses, nalgene water bottles, and bulletproof glass windows. PC showsan increase in impact resistance over a wide range of saturationpressures from 1, 4, and 7 MPa. In FIG. 33, the impact strength of solidhigh-impact polystyrene saturated at 1 MPa and desorbed has an impactstrength lower than the impact strength of the standard material. Theimpact strength of solid high impact polystyrene saturated at 4 MPa anddesorbed has an impact strength lower than the impact strength of thestandard material and of the polystyrene saturated at 1 MPa. High ImpactPolystyrene (HIPS) exhibits a decreasing impact strength with thesaturation pressures. Accordingly, whether the impact strength of amaterial increases or decreases depends on the saturation pressures.

As disclosed above, at saturation pressures of 5 MPa, nano-sized cellswere formed in polyetherimide. A nano-sized cell as defined herein has asize from 0.1 nm to 100 nm. A nanocellular foam has cells that have anaverage cell size from 0.1 nm to 100 nm. In another embodiment, anano-sized cell can have a size from 20 nm to 40 nm. At saturationpressures of 1 MPa, micro-sized cells were formed in polyetherimide. Amicro-sized cell as defined herein has a size from greater than 0.1micrometers (μm) to 100 micrometers (μm). A microcellular foam has cellsthat have an average cell size from 0.1 μm to 100 μm. At saturationpressures of 4 MPa, both nano-sized cells and micro-sized cells wereformed in polyetherimide. These cellular structures are described below.

Referring to FIG. 17, a scanning electron micrograph of across-sectional view of foamed polyetherimide shows nano-sized cellshaving cell diameters in the submicrometer range. Foams havingnano-sized cells can be created using the hot liquid bath or heatedpress method disclosed herein or other heating method. Foams havingnano-sized cells with an average cell size of from 0.1 nm to 100 nm arebelieved to have desirable mechanical properties, such as lighttransmissivity, impact strength, and tensile elongation that aresubstantially the same or higher than that of the starting solidthermoplastic material, and are also believed to be higher than the sameproperties for a microcellular structure. In the particular instance ofpolyetherimide, based on the molecular size calculations forpolyetherimide of 100 nm, the cellular size produced in the nanocellularmaterial creates bubbles much smaller than the estimated molecular size.The circumference of half a bubble would be in the range of 31.4-62.8 nmfor a cell size range of 20-40 nm. This would allow for polyetherimidemolecules to stretch from one side of the cell to the other and stillhave 38-69 nm of molecular length to entangle with molecules on eitherside of the cell. The scale of the cellular size might have drasticeffects on the mechanical properties of these nanocellular forms. Incontrast, microcellular voids of 3 μm would create a half circumferenceof 4.7 μm. At this cellular size, it would take forty-seven 100 nm-sizedpolyetherimide molecules to reach from one side of the cell to theother.

FIG. 18 is a scanning electron micrograph of a cross-sectional view ofmicrocellular polyetherimide with cell diameters in the 3 micrometerrange. Foams having micro-sized cells can be created using the hotliquid bath or heated press method disclosed herein or other heatingmethod. Micro-sized cells are considered to have an average cell sizeranging from greater than 1 μm to 100 μm.

Both the microcellular thermoplastic foams and the nanocellularthermoplastic foams can be created using a solid-state foaming method,such as the heated liquid bath method or any variation of the heatedpress method disclosed herein, wherein the saturation pressure is 1 MPato create microcellular structures, and the saturation pressure is 5 MPato create nanocellular structures. When the structure includesnano-sized features and micro-sized features, the structure is bimodal.Several bimodal cellular structures are possible using the disclosedfoaming methods. To create bimodal foams as disclosed herein, the gasbeing desorbed during the heating process may be allowed to escape fromthe material.

Referring to FIGS. 19 and 20, scanning electron micrographs of across-sectional view of a polyetherimide foam produced using the heatedpress method shows a foam having cells of an average cell size of about4 μm. These micro-sized cells form the primary structure. However, uponcloser inspection of the inner cell walls, it was found that the heatedpress method produced nano-sized features on the cell walls of thelarger microcellular features. Some of the nano-sized features createconnectivity between the larger micro-sized cells. The heated liquidbath or the heated press with means to allow channeling of the gas fromthe surfaces produces these bimodal cellular structures. In oneembodiment, using a thermoplastic material that is saturated at 1 MPasaturation pressure may be used to make the bimodal structurerepresented in FIGS. 19 and 20. The structure may be referred to hereinas an “intrabimodal” cellular structure. It is believed that anintrabimodal cellular structure may be created having micro-sized cellshaving an average cell size of greater than 1 μm to 100 μm and havingnano-sized features having an average size of 0.1 nm to 500 nm.

Referring to FIGS. 21 and 22, scanning electron micrographs of across-sectional view of foamed polyetherimide using a saturationpressure of 4 MPa are shown. Micro-sized cells having a cell size ofapproximately 2 to 3 μm are interspersed among a majority of nano-sizedcells having average cell sizes of less than 1 μm. The heated pressmethod without the use of a breather layer can make this bimodalstructure. This bimodal structure has the majority of cells in the 1 to2 μm range with an evenly distributed secondary structure of cells inthe 10 to 15 μm range. These bimodal cellular structures may be referredto herein as “intercellular” bimodal structures. A bimodal structuresuch as this may have superior mechanical properties to that of foamshaving either of the two cell sizes alone. It is believed that aninterbimodal cellular structure may be created having the smaller cellsforming the primary structure with an average cell size of less than 2μm and the larger cells interspersed throughout the primary structurehaving cells that have an average cell size of 2 μm to 100 μm.

In another embodiment, a microcellular polyetherimide foam can becreated independent of the saturation pressure. At a saturation pressureof 5 MPa and using a breather layer between the hot platens and thethermoplastic material, gas is allowed to escape from the surfaces ofthe material. The resultant structure is a cellular structure havingnano-sized cells. When the material is foamed in the heated presswithout the use of the breather layer, the foam structure hasmicro-sized cells. A breather layer can be a porous cloth that allowsgas to travel from the surface of the polymer to outside of the press.There are advantages of this process over the conventional bath foamingprocess, including: (1) the cellular size is determined independent ofthe saturation step, and (2) at higher pressures, the saturation steprequires less processing time to make micro-sized cells. By saturatingat 5 MPa instead of 1 MPa, the saturation processing time is reduced byapproximately ten days to make microcellular foams. Microcellular foamssaturated at 5 MPa can be created that are equivalent in scale to thatproduced when saturated at 1 MPa. By using this method, the time neededto create microcellular polyetherimide is significantly reduced.

Although polyetherimide has been disclosed as capable of formingnano-sized cells, other thermoplastic polymers are thought to be able toform nano-sized cells. Representative thermoplastic materials that maybe useful in accordance with embodiments of the present inventioninclude amorphous polymers and semi-crystalline polymers. Representativecompounds include, but are not limited to, thermoplastic urethanes,thermoplastic elastomers, polyethylene naphthalate, polyetherimide,polyetheretherketone, polyphenylene, sulfone, polyamide-imide,polysulfone, polyphenylsulfone, polyethersulfone, polyphthalamide,polyarylamide, polyphenylene sulfide, cyclic olefin copolymer,polyphthalate carbonate, polycarbonate, polyvinylidene chloride,polyurethane, polyphenylene oxide, poly(acrylonitrile-butadiene-styrene), polymethylmethacrylate, crosslinkedpolyethylene, polystyrene, styrene acrylonitrile, polyvinyl chloride,polybutylene terephthalate, polyethylene terephthalate,polyoxymethylene, polyacetal, polyamide, polyolefin, polyethylene,polypropylene.

Polyetherimide was selected as a representative thermoplastic materialfor its ability to create a wide range of cell sizes. Polyetherimide hasthe chemical formula C₃₇H₂₄O₆N₂. The monomer molecular weight is 592g/mol. The molecular weight of a polyetherimide polymer averages 30,000g/mol. By dividing the polymer molecular weight from the monomermolecular weight, on average there are 51 repeating monomer units perpolymer chain.

EXAMPLES

Sample Preparation

Samples for testing density, impact strength, light transmissivity,tensile elongation (strain), and a modified press foaming method werecut from a polyetherimide sheet with a backing film attached to thesheet. All samples had a thickness of 0.06 inches or 1.5 mm. Samplesused for a tensile strain test required a secondary operation to cut thesamples into a dog-bone shape from the sample blanks. A foot-operatedshear press was used to achieve the desired sample dimensions. Sampleswere cut using the press with the protective polymer film still attachedto the samples. After the samples were cut, the surface film layer wasremoved. Due to the slight brittle nature of polyetherimide, sampleswere inspected for cracking after the shearing operation. Any sampleswith surface blemishes or internal cracks were discarded.

Samples for impact testing were created following the recommendations ofASTM D5420 “Standard Test Method for Impact Resistance (GardnerImpact).” The standard specifies that the impact sample be at least 1inch greater than the diameter of the support plate hole. According tothe ASTM standard, the minimum sample size for the test apparatus wouldthen be 1.64″×1.64″. Samples measuring 2″×2″ were produced in an effortto reduce the effect of shear cutting inducing undetectable defectsalong the shear edge. Samples for density and light transmissivitytesting were sheared from the raw polyetherimide sheet into 1″×1″samples. The small sample size allowed for easy saturation in pressurevessels and provided the required accuracy.

Samples for tensile testing required a further cutting operation toproduce the required ASTM dog-bone shape. Blanks were cut to 4.5″×1″,followed by a milling operation. The samples for tensile elongationtesting were manufactured according to ASTM D638 Type IV specifications.Samples were machined in batches of ten using a TRAK K3E 2-axis CNC kneemill accurate to within 0.001″. A 0.5″ diameter Swift-Carb™ carbidesteel end mill, rotating at 800 rpm, was used to cut the samples.

Samples used for obtaining the mechanical testing results shown in FIGS.25 to 30 were saturated for at least the time periods shown in Table 1below in accordance with the saturation pressure used. Samples used indensity measurements were foamed at the temperature shown in Table 1below.

TABLE 1 PEI Processing Conditions Used for Mechanical Samples 1 MPaSaturation Pressure Full Saturation Absorption Time 22 days Desorptionto <1% g CO₂/g PEI 17 days Total Processing Cycle Time 39 days RelativeDensity Foaming Temperature (° C.) 75% 181 82.50%   173.5 90% 167 5 MPASaturation Pressure Full Saturation Absorption Time 12 days Desorptionto <1% g CO₂/g PEI 27 days Total Processing Cycle Time 39 days RelativeDensity Foaming Temperature (° C.) 75% 137 82.50%   126 90% 113Saturation of Samples

Samples were wrapped in paper towels to assure that gas is absorbed byall surfaces evenly. The wrapped samples were then placed in a pressurevessel and sealed. Carbon dioxide with a 99.9% purity, supplied byAirgas Norpac, was then delivered to the pressure vessel from a highpressure tank. The saturation pressure was controlled by a PIDmicrocontroller to an accuracy of ±0.1 MPa. Samples were then allowed toabsorb gas over a predetermined amount of time. After the samplesreached full saturation, they were removed from the pressure vessel andallowed to desorb gas before being foamed. The samples were allowed todesorb for 2 minutes before being placed in the foaming bath. Allsamples used to report on the mechanical properties were prepared usinga heated bath process. Samples used in press foaming were placed in afreezer at 0° C. to slow the desorption of gas from the polymer, andthen placed in the heated press. The samples were generally at the samegas concentration before foaming.

Foaming Methods

Two methods for foaming samples were used. A hot oil bath was used tofoam all of the samples used for density, impact strength, lighttransmissivity, and tensile characterization. A modified heated pressmethod was used to foam samples for foam characterization. Heated pressfoaming is advantageous to create flat specimens and to characterize thecellular structures produced by the heated press foaming method.

For oil bath foaming, following a 2-minute desorption period atatmospheric pressure and room temperature, the samples were placed intothe METTLER balance to measure gas concentration. At 2.5 minutes, thesamples were then placed into a temperature controlled ThermoHaake B5hot silicon oil bath and foamed for 2.5 minutes. At 5 minutes fromdepressurization, the samples were removed from the oil bath. Any excessoil was removed from the surface of the samples and the samples wereallowed to cool to room temperature. A wire cage was used to house thespecimen to keep the sample submerged in the oil bath.

A heated press apparatus included an upper and lower heated platen, ahydraulic pressure cylinder, and a control system. Various processingparameters can be varied via the control system. In heated pressfoaming, the platens were heated to the desired temperature with theplatens in the closed position. Once the platens are at temperature, asample is removed from the pressure vessel and allowed to desorb forseveral minutes, generally 2 to 4 minutes, while being transported tothe press. However, the desorption times were generally insufficient tosubstantially change the concentration of gas. Generally, the samplescontained substantially the same gas concentration. The heated platensare opened and the sample is placed between the upper and the lowerplatens at about the center of the platens. The platens are then closedso that the heated surface of the upper platen touches the upper surfaceof the sample and the heated surface of the lower platen touches thelower surface of the sample. See, for example, FIG. 5. Both the upperand the lower platen make contact with the sample and apply pressure tothe upper and lower surfaces of the sample. The foam will then pushagainst the pressure created by the platens in order to foam. Heatingoccurs on both surfaces of the sample through conduction.

After foaming, the samples were allowed to desorb to a minimum gasconcentration before the mechanical properties of the samples weretested.

Desorption

To determine the time required to reach a minimum gas concentrationbefore mechanical testing, desorption tests were performed on sampleswithout undergoing foaming. The samples were removed from the pressurevessel and allowed to sit at room temperature at atmospheric pressure.Periodic weight measurements were taken to record the amount of gasdissolved in the sample using a METTLER AE240 balance. After the sampleshad desorbed carbon dioxide to a concentration less than 0.01 gCO₂/gPEI, they were considered “fully desorbed.” The threshold gasconcentration for performing mechanical testing is when theconcentration is equal to or less than 10 mg carbon dioxide per gram ofpolyetherimide. The minimum desorption times for the various saturationpressure are listed in Table 2 below.

TABLE 2 Desorption Times Before Mechanical Testing Saturation PressureRequired Desorption Time (Hours) 1 400 2 550 4 650 5 650

Foam and non-foamed (virgin) samples of polyetherimide were desorbed ofgas before being tested for mechanical properties. The threshold gasconcentration for mechanical testing was chosen as less than 10 mgcarbon dioxide per gram polyetherimide. It was not necessary to allowdesorption of foams used for structure characterization. Desorptiontimes required for foams will be less than the values in Table 2 becausethe foaming process releases gas from the polymer to create the cellularstructure and also because gas desorbs faster in a cellular structure.Foamed samples desorbed according to the times in Table 1 will then havea much lower gas concentration than the solid samples. Nevertheless,regardless of the minimum desorption times in Table 2, the foamedsamples used in mechanical testing were allowed to desorb for at least700 hours. After 700 hours, the gas concentration is essentiallyconstant regardless of the saturation pressure used.

Density Measurement

Density evaluation was performed according to ASTM standard D792. Theflotation weight loss method uses distilled water as the liquid. Thesample is first weighed “dry,” and then the sample is placed below thesurface of the water and weighed again. Care was taken to avoid thatthere were no gas bubbles attached to the surface of the sample duringthe “wet” weight measurement. The equation used to calculate the densityof the polymer sample is:

$\begin{matrix}{D = {\left( \frac{W_{d}}{W_{d} - W_{w}} \right) \cdot D_{w}}} & (1)\end{matrix}$

where,

D=density of the sample

W_(d)=dry weight

W_(w)=wet weight

D_(w)=density of distilled water (taken as 0.9975 g/cm³)

Density is reported as relative density or void fraction. Relativedensity is the density of the foamed material divided by the density ofthe unfoamed material. Void fraction is defined as one minus therelative density. Both relative density and void fraction are expressedas a percentage. For example, a material with 60% relative density meansthat the total volume of the foamed sample is 60% polymer and 40% air.

Light Transmissivity Measurement

As used herein, transmissivity is the fraction of light that passesthrough a material for a specified wavelength. Transmissivity iscalculated as the ratio of the intensity of light that passes through amaterial divided by the intensity of the light source. Thetransmissivity value is inversely related to the opacity of the sample.An opaque sample will not allow light to pass through; therefore, itwill have a low transmissivity.

FIG. 23 is a schematic circuit diagram of a device that was designed andconstructed to test the light transmissivity of materials. Theinstrument depicted in FIG. 23 emits a specific wavelength of light at asample and then measures the amount of light that passes through thesample with a photoresistive transistor detector 1502. The voltage dropacross the series resistor 1504 is measured and compared to the voltagedrop when no sample exists between the emitter 1506 and detector 1502.The ratio of these values is the transmissivity. The circuit illustratedin FIG. 15 was built on a solderless breadboard. A cardboard cover wasconstructed to shield ambient infrared light from entering the detector1502 during measurements. The emitter 1506 and detector 1502 werepurchased as a matched set of near-infrared light emitting diodes(LEDs). Near-infrared light exists just outside the visibleelectromagnetic spectrum. The LED emitter 1506 had a peak wavelength of940 nm. The procedure to measure the transmissivity of a cellular sampleis as follows. The cover is placed over the emitter 1506 and detector1502 and the V_(out) voltage measured between points 1508 and 1510 isrecorded. After recording the voltage, the cover is removed and thesample is placed inside the cover, perpendicular to the length andbetween two sides of the cover, making sure that no gaps exist betweenthe cover and the sample. For example, the sides of the cover may berepresented by the parallel lines of the letter “H” such that the sampleis represented by the middle perpendicular line. The cover can be acardboard box with five sides and lacking the bottom side. The insidedimensions of the cover are matched to the dimensions of the samplebeing tested. The cover with the sample is then placed over the emitter1506 and detector 1502 so that the sample lies in the path of lightbetween the emitter 1506 and detector 1502. The V_(out) voltage isrecorded and the transmissivity is calculated by dividing the samplevoltage by the voltage without the sample.

Impact Strength Measurement

Impact strength measurement was performed according to ASTM D5420. Theimpact strength is a measurement of energy required to break or crack aflat polymer sample by the impact of a falling weight.

The procedure to measure the impact strength of microcellular andnanocellular polyetherimide foam required foaming 2″×2″ samples. TheASTM standard calls for a minimum of 20 impact samples for sufficientresults, assuming the mean failure height is known. The testing of thepolyetherimide samples required 26 samples, including 6 to estimate themean failure height and 20 to perform measurements.

Measurement of Tensile Strain

Tensile testing generally requires the application of a graduallyincreasing uniaxial stress until the propagation of a single crackcauses failure. Samples for tensile testing were manufactured to ASTMD638 Type IV specifications. Testing of these samples also followed ASTMD638. Tensile testing was performed on an Instron 5585H. In thisapparatus, serrated jaws hold the tensile samples. A constant crossheadrate was used to control the amount of stress applied to the polymersamples. The majority of testing was conducted with a crosshead rate of10 mm/min. Tests were also conducted at a rate of 50, 100, and 200mm/min. An extensometer with a gage length of 25 mm was employed tomeasure the initial strain until approximately half the yield stress.After removing the extensometer, strain was recorded from the extensionon the Instron crosshead.

Microstructure Characterization

The characterization of microcellular and nanocellular polyetherimidestructures was performed by imaging the structures with a scanningelectron microscope (SEM). All images were taken on a digital FETSiriron scanning electron microscope. Samples were first scored with arazor blade and freeze fractured with liquid nitrogen. Samples were thenmounted in metal stages and the imaged surface was sputter coated withAu—Pd for between 20 to 60 seconds. Accelerating voltages varied between2 to 10 kV for imaging and both “high resolution” and “ultra-highresolution” detectors were used, depending on the size of themicrostructure.

A saturation pressure of 1 MPa was used to create foams having amicrocellular structure and a saturation pressure of 5 MPa was used tocreate foams having a nanocellular structure. FIG. 16 shows the relativedensity as a function of foaming temperature for saturation pressures of1 MPa and 5 MPa for polyetherimide foam. The cellular structures wereimaged as described above to identify the average cell sizes for thedensities used in mechanical testing.

Microcellular samples produced in the density foaming experiments rangedfrom 96.5 to 28.7% relative density. High density samples were analyzedand average cell size was calculated by averaging the cell size of atleast 30 cells. The average cell size of a 91.5% relative density samplewas 2.5 μm (see FIG. 18) and the average cell size for a sample ofrelative density 70.2% was 3.5 μm.

The images of lower relative density microcellular samples showinteresting structures below a relative density of approximately 56%.See, for example, FIG. 19. These samples exhibited microcellularstructure but, upon closer inspection, the inner cell walls shownano-sized features. The nano-features can best be described assegmented sections of the cell walls where, during bubble growth,polyetherimide molecules remain entangled and bridge the slight radiusof the cell wall. FIGS. 19 and 20 support this description of thestructure and also show that these nano-features are reduced when cellsadjacent to the cell in question push to create flat sections on thecell wall. These flatter sections of the cell show less nano-featuresthan other sections where there is more curvature. In addition to thesenano-features, samples with lower densities show that thesenano-features create nano-interconnectivity between one microcell toanother. The interconnectivity does not take the shape of circular cellsbut appears as a network or webbing of stretched molecules.

In order to image nanocellular samples using SEM, Au—Pd coating timeswere reduced to avoid covering the cells. Nanocellular samples had adensity range of 70-90% relative density and had cells averaging 20-40nm, depending on the density. See, for example, FIG. 17 at 69.9%relative density.

Foam Structure Characterization at 4 MPa

Samples treated at a saturation pressure of 4 MPa were formed at varioustemperatures using the heated oil bath method to study the structure anddensity of polyetherimide at various foaming temperatures. SEM images ofthe microstructure of these foam samples are shown in FIGS. 21 and 22.Average cellular sizes ranged from 250 nm to 1 μm. The foamcharacterization experiments carried out at 4 MPa confirm that in therange of 4-5 MPa, cellular size decreases from 1 μm to 60 nm, dependingon foaming temperature.

Internal Blistering

Over the course of experimentation, it was observed that a few samplesdeveloped large internal blisters. The formation of large internalblisters in polyetherimide should not be confused with the surfaceblisters usually observed during the solid state method. Normal surfaceblistering usually appears when the integral skin of a polymer becomestoo weak to support the internal foaming pressure during processing.Surface blistering often occurs when foaming at high temperatures in theattempt to create very low density foam. In contrast to surfaceblisters, internal blisters originate directly from the center of thesample. In the first 20-30 seconds of foaming, the samples that willlater develop internal blisters begin to curl. Samples that exhibitextreme curling during the first half minute of foaming develop internalblisters by the end of the foaming process. Many samples that do notdevelop blisters will sometimes develop a curl, but this curling iscreated during the entire 2.5-minute foaming process, not solely in thefirst 20-30 seconds. During the last 30 seconds of foaming, the internalblisters begin to appear. The internal blisters will often pop in theoil bath when the material creates the large internal cavity of gas. Thesolid skin and foamed sections around a gas pocket yield to the highpressure of the gas. After removal from the oil bath, defective samplesretain the large curvature and the large internal blisters producedwhile foaming. Internal blisters appeared in samples of all dimensions,including the dog-bone shaped samples for tensile testing.

One possible cause for the internal blistering is a weak mid-plane inthe center of the material traveling parallel to the material's surface.The weakened mid-plane could be caused by a defect in the raw materialor is a function of the solid-state foaming process. Crack propagationstarts perpendicular to the surface, then abruptly changes direction tothe mid-plane parallel to the sample surface.

Creation of Flat Foams without Internal Blistering

Because the heated oil bath foaming method often creates slightcurvature with larger sample sizes, a modified constrained press foamingmethod was used to create flat microcellular and nanocellularpolyetherimide.

Referring to FIG. 4, a schematic illustration of a prior art constrainedpress foaming process is illustrated. In this process, shims 406 and 408can be placed between the upper 402 and lower 404 platen that determinethe thickness of the foam 410. The upper 402 and lower 404 platens areprevented from closing beyond the dimension of the metal shims 406 and408. Heat transfer from the platens 402 and 404 to the sample 410includes conduction by direct contact of one surface of the sample to aheated platen and by convection of heat from a second platen to theopposite surface of the sample.

Experiments using the prior art press foaming method of FIG. 4 wereperformed on a variety of sample sizes including 3″×3″, 3″×6″, and6″×6″. Samples were saturated at 1 MPa until full saturation wasreached, based off saturation times presented above. Upon fullsaturation, samples were removed and placed in a freezer set to 0° C. toslow the process of desorption. Two foaming temperatures were exploredin the constrained foaming process including 197° C. and 210° C. Sampleswere foamed a total of 3 minutes, whereupon the cooling systems wereturned on to cool the platens for sample removal. The first foaming runsfollowed the proposed constrained foaming process developed by Nadellaet al. (Nadella, K., Kumar, V., Li, W., “Constrained Solid-State Foamingof Microcellular Panels,” Cellular Polymers, p. 71, 2005). Theconstrained foaming method of Nadella et al. sets a fixed thicknessbased on the estimated thickness after foaming is completed by the useof shims. Samples are slid between the platens and allowed to foam tothe top platen. See FIG. 4.

Two variations of the conventional constrained foaming process ofNadella et al. were explored. Polyetherimide samples exhibited a largescrap rate due to internal blistering when allowing to free foamaccording to the conventional method. One variation illustrated in FIG.5 included removing the shims and closing the press until both heatedplaten surfaces touched the surface of the sample. Two modes ofoperation are possible where the first and second platens 502 and 504apply a force on respective upper and lower sides of the sample 510. Inthis mode, heat transfer to the sample 510 is by direct conduction ofheat from the upper 502 and lower 504 platens. Samples grew in volume bythe driving force exerted on the platens from the expanding gas. In thesecond variation illustrated in FIG. 6, nylon composite manufacturingbreather layers 612, 614 were added between the sample 610 and theplaten surfaces 602, 604.

The density of the samples foamed by the conventional constrainedfoaming process matched that of the samples produced by the oil bathprocess. However, 100% of the samples developed large internal blisters.The creation of internal blisters occurred early in the over-all foamingprocess before the foamed sample reached the top platen surface. Onesolution was to close the platens to touch the surfaces of the samplewithout shims and allowing the foam to expand against the force of theplatens as illustrated in FIG. 5. This modification resulted in improvedquality, and the scrap rate decreased. Despite this improvement, manyparts still had a number of irregularities. Only a small fraction of the“good” foams had a smooth surface. Many foams foamed without the use ofshims had irregular volumetric expansion during foaming.

The effect of closing the platens entirely on the saturated samplecreated an overall flatness of the samples with many samples having asmooth surface. Scrap parts of this process had small internal blistersin addition to indents on the surface. It is assumed that the surfaceindents were caused by the collection of gas escaping from the surfaceof the polymer during foaming. To avoid the formation of internalblisters and surface deformations, a breather layer was placed betweenthe sample surface and platen surface to allow for gas to escape as thegas desorbed from the sample during foaming as illustrated in FIG. 6. Abreather layer was obtained from a composite manufacturing lab andincluded a woven nylon fabric having a thickness of approximately 0.5mm. The use of a breather layer created a surface texture on thesamples, but reduced the amount of scrap due to internal blistering tobelow 25%. Allowing the gas to escape from the surface of the polymerunobstructed seems to decrease the scrap rate. Additionally, applyinggreater pressure on one area and reduced pressure across another areacreates a pressure gradient that prevents foaming in the higher pressurearea. Samples were produced that had sections of foam and sections ofunfoamed, solid material.

Mechanical Testing

Saturation pressures of 1 MPa and 5 MPa were chosen to createmicrocellular and nanocellular samples, respectively. Saturated sampleswere then placed in the heated oil bath for foaming into cellularstructures. A comparison of mechanical properties of cellular structuresranging from 20 nm to 3 μm could then be studied. Experiments toevaluate the mechanical properties were run using the processingparameters for absorption and foaming temperatures shown in Table 1. Thedesorption time was set at 700 hours regardless of saturation pressure.The relative density range between 75-90% was chosen to reduce theeffect of internal blistering. Three variations of density were chosenat 75%, 82.5%, and 90% relative density.

Light Transmissivity Results and Discussion

FIG. 24 shows the infrared light transmissivity as a function ofrelative density for microcellular and nanocellular foam samples. Thedata shown in FIG. 24 indicate that transmissivity is a function of thecell size. The transmissivity value shown in FIG. 24 is also a functionof the thickness of the sample. Both cell size and sample thickness werenot held constant over the range of void fractions reported. Thethicknesses of the samples increased with void fraction, as can beexpected, although both microcellular and nanocellular thickness grew atrelatively the same rate. The actual values of transmissivity reportedin terms of the rate and range of transmissivity change cannot becompared to data outside of this experiment, but can still be used as acomparison between the two sets of samples tested. Cellular sizes alsoincreased within the reported micro and nano ranges with increasing voidfractions. Although this will alter the absolute value oftransmissivity, the comparison between the micro and nano ranges canstill be made. Both the increase in thickness and cell size willprematurely decrease the absolute value of transmissivity over theranges investigated.

The optical behavior of the nanocellular structures can be explained inpart by a series of equations governing electromagnetic scatteringdeveloped by Lord Rayleigh. These equations demonstrate that theintensity of scatter is dependent on the wavelength of light and evenmore so by the diameter of the scattering particle. The dependence onthe wavelength shows that certain wavelengths are more easily scattered.In the visible spectrum, violets and blues are more easily scatteredthan red wavelengths.

A color change in the nanocellular plastic is hard to identify due tothe amber color of the material, but when the sample is held to a whitelight source, the light that passes through the material is red incolor. Subjective to the observer is the slight hue of blue color in thehigh density nanocellular samples. It is then hypothesized that if thestarting polyetherimide material had no color, nanocellular foamedpolyetherimide would then be slightly blue in color. This provides amethod to create color in a polymer without the addition of extrapigments or pigmentation agents, or of a method to filter light of acertain wavelength.

Based on the data of FIG. 24, it can be concluded that there is adifference between the transmissivity of microcellular polyetherimideand nanocellular polyetherimide. Cell size has a dramatic effect on theamount of light scattering through the material. The transmissivity ofinfrared light as a function of void fraction shows that nanocellularmaterial allows more light to pass through a sample. This experimentsupports the idea that the optical transparency of foams can be adjustedby controlling the size of the cells. When the cells are sufficientlysmall, the thermoplastic foam may be essentially 100% transparent.

Impact Strength Results and Discussion

Standard unfoamed unsaturated polyetherimide samples were first testedto benchmark the behavior of polyetherimide. The mean energy forbreakage was calculated according to ASTM D5420. Foam samples withdensities of 75, 82.5, and 90% relative density were then created withmicrocellular and nanocellular structures. After the foamed samples wereallowed to desorb, they were impact tested. Impact results for foams canbe displayed in many ways. The units presented for impact energy can bein joules, joules per millimeter, and joules per millimeter per density.

Impact strength is dependent on the thickness of the sample. The rawvalue for impact energy can be divided by the average thickness of thesamples to get impact energy per thickness. In increasing the voidfraction of a sample, the overall dimensions of the sample will enlarge.The thicknesses of the samples are then a function of the beginningsample thickness and the degree of foaming. FIG. 25 shows a graph of theimpact energy compensated for the change in thickness during foaming.FIG. 25 shows the impact strength in joules/mm as a function of voidfraction for the three foam densities, virgin polyetherimide and atreated solid polyetherimide that was treated by saturating the samplewith gas and then allowing desorption of the gas.

From FIG. 25, a difference in the impact strength between solid (squaredata point) and foam (circle and diamond data points) polyetherimide, aswell as a difference between the two cellular structures, is evident.Additionally, the trends of the least squares linear fit show that thereis a convergence of impact strength between the two cellular structures.Both cellular structures and virgin polyetherimide all have the sameimpact strength between void fractions of approximately 25-30%. It isinteresting to note that the nanocellular 10% void fraction samplereached the limits of the testing apparatus such that 50% of the samplesbroke at the limit value. According to the ASTM standard, the meanbreakage is defined where 50% of the total samples break.

The impact strength of microcellular polyetherimide shows an increase instrength with an increase in void fraction in FIG. 25.

In another experiment, polyetherimide samples were prepared bysaturating the samples at a pressure of 5 MPa with carbon dioxide andthen allowed to fully desorb of gas. The impact samples were then testedaccording to ASTM D5420. FIG. 25 also shows the impact strength of solidsamples of polyetherimide having been saturated and then desorbed of gas(triangle data point). The saturated and desorbed solid polyetherimidesample had the highest impact strength of all of the samples tested.Unlike the nanocellular samples, saturated and desorbed samples did nothave any breakage. This indicates that the actual value for the impactenergy of the saturated and desorbed polyetherimide is significantlyhigher than what is reported. The value shown in FIG. 25 is slightlyhigher than the 10% void fraction nanofoam because the saturated anddesorbed sample did not grow in thickness. Since saturated and desorbedpolyetherimide had no observable breakage, it is hypothesized that theactual impact strength would be closer to where the linear best-fit lineintersects the vertical axis in FIG. 25. Impact testing showed thatsaturation and desorption of carbon dioxide gas has an effect on theimpact strength of the polymer. Saturated and desorbed polyetherimidesamples had the highest impact resistance overall and saw a doubling ofimpact resistance to that of neat polyetherimide.

Tensile Elongation (Strain) Results and Discussion

The tensile experiment tested virgin samples, saturated and desorbedsamples, microcellular samples, and nanocellular samples ofpolyetherimide. Foamed samples were tested at three densities including90%, 82.5%, and 75% relative density. All tensile characterization testswere performed at a strain rate of 10 mm per minute unless otherwisenoted. The results of the virgin and 5 MPa saturated and desorbedpolyetherimide tensile samples are shown in FIG. 26. Tensile results aredisplayed by a solid curve representing the cumulative stress-straincurves for each condition with fracture points indicating the point ofindividual sample fracture. Referring to FIG. 26, the effect ofsaturation and desorption with 5 MPa carbon dioxide is a lowering of theyield strength. The strain at break observed overlaps for both sets ofsamples and shows that saturation and desorption has little to no effecton strain at break.

Microcellular and nanocellular polyetherimide tensile samples were thentested. The data plots are shown in FIGS. 27-29 with the fracture pointsof each sample shown as an open circle for microcellular structures andan open triangle for nanocellular structures.

The data collected from the tensile tests was then used to calculate themean/standard deviation of strain at break.

Mean and Standard Deviation of Strain at Break and Discussion

The strain at break is the measure of stretch the gage section of thetensile dog-bone sample undergoes during the course of the tensile test.Often it is desirable to have a material that has a high value of strainat break because the stretching of the polymer allows for energyabsorption and allows for less drastic failure mode. The difference inmean and standard deviation of the tensile strains between themicrocellular structures and nanocellular structures is visible in thestress-strain curves illustrated in FIG. 30. FIG. 30 shows a differencebetween the means of the strain at break as well as a difference in sizeof the deviation bars. The nanocellular samples exhibit greater tensilestrain (elongation) than microcellular samples. In addition,nanocellular samples have a tighter cluster of break points. The smallerthe variation in break points, the more reliable the foam is undertensile loading. Shown in the stress-strain curves, three microcellularsamples of 75% relative density did not even strain past the yieldpoint. The higher reliability of the nanocellular structure couldprovide greater value over microcellular polyetherimide to a designengineer by providing a predictable increased energy absorption andvisual plastic deformation before breakage.

The strain behavior of the samples gives some insight into themicro-molecular behavior during tensile testing. During the plasticdeformation region beyond the yield stress, amorphous polymers, such aspolyetherimide, allow their molecules to stretch. It appears that thecell size of the microcellular samples hampers the stretching of theamorphous molecules. Nanocells allow for much more stretching. The sizeof the nanocells is within the range of the length of a single polymerchain. Many polymers like that of polyetherimide have molecular chainsof lengths in the tens to hundreds of nanometers. The length of themolecules as compared to the size of the cells allows for a singlemolecule to stretch from one side of a nanocell to the other and beyond.This nanocellular structure may be the cause of the tensile trendsobserved.

The last quality measure from the stress-strain curve is the toughness.The toughness is a combination of the stress and strain at break for thesamples observed. Since nanocellular structures allow for greaterstresses and strains, the toughness of the nanocellular material isvastly improved over microcellular polyetherimide. The largest increaseof nanocellular polyetherimide over microcellular appeared at 75%relative density in that nanocellular material was 3.8 times tougherthan microcellular material of the same density.

The mean strain at break was increased for nanocellular polyetherimideand the standard deviation of the strain at break was significantlydecreased. This allowed for a large increase in the toughness of thenanocellular material as it is a function of stress and strain. It ishypothesized that nanocellular polyetherimide has more strength thanmicrocellular polyetherimide due to the ratio of cell size to molecularlength. One molecule is able to bend around the circumference of asingle cell and entangle with other molecules on both sides of the cell.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

1. A method for making a cellular thermoplastic material, comprising:obtaining a thermoplastic material impregnated with gas, wherein thematerial includes a surface defining a length/width plane and thematerial has a thickness; placing the material within a press; applyinga three to the material with the press normal to the surface, whereinthe force produces friction between the surface and the press togenerally cause expansion in the thickness dimension and preventexpansion in the length/width plane.
 2. The method of claim 1, whereinmore than one thermoplastic material is placed within the press.
 3. Themethod of claim 1, wherein more than one thermoplastic material issaturate with gas, wherein a porous media is interleaved betweenthermoplastic materials.
 4. The method of claim 1, wherein the foamthermoplastic material is greater than 3 mm in thickness.
 5. The methodof claim 1, wherein the foam thermoplastic material is 6 mm in thicknessor greater.
 6. The method of claim 1, further comprising: heating thepress to form cells in the material; and channeling the gas desorbingfrom the material away from the material through a gas channeling meansthat is provided within the press to provide a cellular materialsubstantially free of internal blistering and surface deformations.
 7. Amethod for making a composite structure including a thermoplastic foam,comprising: obtaining a thermoplastic material and a facesheet;impregnating the thermoplastic material with a gas; allowing thegas-impregnated thermoplastic material to desorb gas at the surfacesthereof; applying an adhesive on at least one surface of thegas-impregnated thermoplastic material or the facesheet or both; placingthe facesheet on the thermoplastic material; placing the gas-impregnatedthermoplastic material with adhered facesheet within a press; andheating the press to cure the adhesive, create a cellular structure inthe thermoplastic material impregnated with gas, and form a thinintegral solid skin at the surfaces of the thermoplastic material thatare desorbed of gas.
 8. The method of claim 7, further comprisingapplying an adhesive to a second surface of the gas-impregnatedthermoplastic material or second facesheet and placing the secondfacesheet on the second surface.
 9. The method of claim 7, wherein thepress is heated to a temperature to cure the adhesive.
 10. The method ofclaim 7, further comprising selecting one of saturation pressure,saturation time, or desorption time to control the density of thecellular structure in the thermoplastic material.